Device and method for photovoltaic power generation using optical beam uniformly condensed by using flat mirrors and cooling method by direct contact

ABSTRACT

Disclosed is technology pertaining to a device and method for photovoltaic power generation using a photoelectric effect. The device for photovoltaic power generation includes: a frame; a photovoltaic panel arranged on the frame such that a light receiving surface is perpendicular to an incident direction of sunlight with the back thereof against the sun; two or more flat mirrors which are arranged on the frame and reflect the incident sunlight to the light receiving surface of the photovoltaic panel; a rotation mechanism for tracking the sun such that the light receiving surface of the photovoltaic panel is perpendicular to the incident direction of the sunlight by rotating the frame in a biaxial direction; and a supporting structure for supporting the frame and the rotation mechanism.

PRIORITY CLAIMS AND CROSS-REFERENCE TO RELATED APPLICATION

This patent document is a continuation application of a PatentCooperation Treaty (PCT) application number PCT/KR2014/005570 entitled“PHOTOVOLTAIC POWER GENERATION DEVICE AND METHOD USING OPTICAL BEAMUNIFORMLY CONDENSED BY USING PLANE MIRRORS AND COOLING METHOD BY DIRECTCONTACT”, filed on Jun. 24, 2014, which claims priority from and thebenefits of Korean Patent Application No. 10-2013-0073282, filed on Jun.25, 2013. The entire disclosures of the above applications areincorporated by reference as part of this document.

TECHNICAL FIELD

This patent document relates generally to a device and method forphotovoltaic power generation which are commercially available. Moreparticularly, some embodiments relate to photovoltaic power generationtechnology that combines light condensing, sun tracking, and cooling ofphotovoltaic panels to reduce the cost of photovoltaic power generationand makes it possible to compete with existing power generation such asthermal power generation or nuclear power generation without relying onsubsidies.

BACKGROUND

It has been more than a century since the photoelectric effect wasdiscovered by Einstein, and half a century since a solar cell developedby the National Aeronautics and Space Administration (NASA) of theUnited States was used in artificial satellites. However, powergeneration projects that are known to be based on the solar cell for afew decades have not yet been successful as profitable self-supportingprojects. The primary reason is the high price of a photovoltaic panel.It was reported on April, 2013 that the photovoltaic panel can besupplied at a price of about $1 per watt (W). In the case of Korea, itis reported that a supply price of electric power is about $0.1/KWh. Apurchase price of the photovoltaic panel required for 1 MW-classphotovoltaic power generation plant and an annual electricity productioncapacity (electric power sale) are compared as follows (in the case ofKorea, it is reported that average photovoltaic power generation can beconducted using sunlight of 1 KW/m² for about 3.5 hours per day onaverage when annualized.).

Price of 1 MW-class photovoltaic panel: $1,000,000 ($1/Watt)

Annual electricity production capacity of 1 MW-class photovoltaic panel(price of annually produced electric power): 1 MW×3.5 h×365 days=1,2775MWh/year ($127,750/year)

In detail, in view of the fact that income obtained by selling electricpower for 7.8 years should be invested for only an expense required topurchase the photovoltaic panel and other incidental expenses (it isreported that, excluding a land cost, an expense about equivalent to theprice of the photovoltaic panel is invested), a photovoltaic powergeneration industry is not very attractive at this point. It is reportedthat some districts, such as Italy, Hawaii, etc. in which a price ofcommercial electric power is highly fixed and which have an abundance ofsolar radiation, approach grid parity. Grid parity refers to a point intime at which the price of commercial electric power is equal to thecost of photovoltaic power generation. In the case of Korea, the supplyprice of electric power is relatively low, and it is more difficult toachieve the grid parity.

SUMMARY

Various implementations of the disclosed technology are to reduce a costof photovoltaic power generation to allow a photovoltaic powergeneration project to be commercially conducted without a support policyfrom public sectors. Some implementations of the disclosed technologyprovide a method and device capable of producing much electric powerfrom a photovoltaic panel with a fixed area.

In one aspect, a device for photovoltaic power generation is provided toinclude a frame; a photovoltaic panel disposed on the frame; two or moreflat mirrors disposed on the frame; a rotation mechanism configured torotate the frame biaxially to track sunlight; and a supporting structureconfigured to support the frame and the rotation mechanism, wherein thephotovoltaic panel and the flat mirrors are disposed on the frame tomeet a predetermined relationship, said predetermined relationshipincluding relations that, in an orthogonal coordinate system whosereference lines vary to allow a unit vector in a traveling direction ofsunlight to be (0, 0, 1) when the rotation mechanism normally tracks thesun, (i) a z-axial component of a unit vector in a direction normal to alight receiving surface of the photovoltaic panel has a positive value,and (ii) a z-axial component of a unit vector in a direction normal to areflective surface of each of the flat mirrors has a negative value toallow the sunlight reflected by each of the flat mirrors to be uniformlyincident upon the light receiving surface of the photovoltaic panel,wherein the photovoltaic panel and the flat mirrors are disposed on theframe such that projected images of the flat mirrors on an x-y plane ofthe orthogonal coordinate system are arranged around a projected imageof the photovoltaic panel on the x-y plane of the orthogonal coordinatesystem in a matrix pattern, wherein an x axis length (M_(x) ^(i,j)) anda y axis length (M_(y) ^(i,j)) of an arbitrary one (M^(i,j)) of the flatmirrors are set to meet the conditions: M_(x) ^(i,j)≧p_(x) cos θ_(x)^(i,j)+δ_(x) ^(i,j) and M_(y) ^(i,j)≧p_(y) cos θ_(y) ^(i,j)+δ_(y)^(i,j), where (i) p_(x) and p_(y) represent an x-axis length and a-yaxis length of the photovoltaic panel, respectively, (ii) δ_(x) ^(i,j)and δ_(y) ^(i,j) represent a minimum x-axis marginal length and aminimum y-axis marginal length of the photovoltaic panel, respectivelyas required to offset arrangement errors of the photovoltaic panel andthe flat mirror (M^(i,j)), an operational error of the rotationmechanism, and vibration to be caused by an external force includingwind, and (iii) θ_(x) ^(i,j) and θ_(y) ^(i,j) are obtained from theequations: tan 2θ_(x) ^(i,j)=d_(x) ^(i,j)/l^(i,j) and tan 2θ_(y)^(i,j)/l^(i,j) where a vector (d_(x) ^(i,j), d_(y) ^(i,j), l^(i,j)) is adisplacement vector from a center of the light receiving surface of thephotovoltaic panel to a center of a reflective surface of the flatmirror (M^(i,j)) in the orthogonal coordinate system, and wherein therotation mechanism is further configured to rotate the fame according toa change in position of the sun such that the predetermined relationshipbetween the photovoltaic panel and the flat miners is maintained.

In another aspect, a device for photovoltaic power generation isprovided to comprise: a fame; a photovoltaic panel disposed on the fame;two or more flat mirrors disposed on the frame; a rotation mechanismconfigured to rotate the fame biaxially to track sunlight; and asupporting structure configured to support the fame and the rotationmechanism, wherein the photovoltaic panel and the flat mirrors aredisposed on the frame to meet a predetermined relationship, saidpredetermined relationship including a condition that, in an orthogonalcoordinate system whose reference lines vary to allow a unit vector in atraveling direction of sunlight to be (0, 0, 1) when the rotationmechanism normally tracks the sun, (i) a z-axial component of a unitvector in a direction normal to a light receiving surface of thephotovoltaic panel has a positive value, and (ii) a z-axial component ofa unit vector in a direction normal to a reflective surface of each ofthe flat mirrors has a negative value to allow the sunlight reflected byeach of the flat mirrors to be uniformly incident upon the lightreceiving surface of the photovoltaic panel, wherein said device furtherincludes a cooling system configured to cool the photovoltaic panel,said cooling system including a fluidic coolant that is in directcontact with surfaces of the photovoltaic panel, and wherein therotation mechanism is further configured to rotate the frame accordingto a change in position of the sun such that the predeterminedrelationship between the photovoltaic panel and the flat mirrors ismaintained.

In some implementations, the photovoltaic panel is disposed on the framesuch that the unit vector in the direction normal to the light receivingsurface of the photovoltaic panel is (0, 0, 1). In some implementations,an optical filter to selectively transmit light of a wavelengtheffective for photoelectric conversion by the photovoltaic panel ismounted on at least one of the reflective surface of each of the flatmirrors or the light receiving surface of the photovoltaic panel. Insome implementations, the sun tracking system comprises first rotatingmeans configured to rotate the fame in a vertical direction and secondrotating means configured to rotate the frame in a horizontal direction.In some implementations, the fame is configured not to block thesunlight on an optical path from the sun to the photovoltaic panel viathe flat mirrors. In some implementations, a variable number of flatmirrors among the flat mirrors are in use for light condensing accordingto an intensity of the sunlight before light condensing in order toobtain a reflected light condensed to a proper level. In someimplementations, the device further comprises a supplementary panellocated on a rear surface of the photovoltaic panel, wherein thesupplementary panel is disposed on the fame such that a unit vector in adirection normal to a light receiving surface of the supplementary panelis (0, 0, −1). In some implementations, the cooling system comprises: asealed housing, at least a portion of at least one surface of the sealedhousing being open; a transparent window configured to cover the openportion of the housing to seal the housing and transmit incident lightto be incident upon the light receiving surface of the photovoltaicpanel; and a holder configured to fix the photovoltaic panel to bedisposed within the housing at a predetermined distance from an internalsurface of the window, wherein the fluidic coolant is filled in thehousing to wrap and cool the photovoltaic panel.

In another aspect, a cooling system for cooling a photovoltaic panel isprovided to comprise: a sealed housing, at least a portion of at leastone surface of the sealed housing being open; a transparent windowconfigured to cover the open portion of the housing to seal the housingand transmit incident light to be incident upon a light receivingsurface of the photovoltaic panel; a holder configured to fix thephotovoltaic panel to be disposed within the housing at a predetermineddistance from an internal surface of the window; and a fluidic coolantfilled in the housing to wrap and cool the photovoltaic panel.

In some implementations, a plurality of valves or holes are provided ona portion of the housing to allow the fluidic coolant to flow into andout of the housing and allow electric wires to be connected thereto. Insome implementations, the cooling system further comprises a circulatorconfigured to cause forcible convection of the coolant in the housing.In some implementations, the window and the photovoltaic panel areremovably coupled to the housing and the holder, respectively. In someimplementations, an optical filter to selectively transmit light of awavelength effective for photoelectric conversion by the photovoltaicpanel is provided on the window of the cooling system. In someimplementations, the fluidic coolant of the cooling system comprises anoptical filtering material or dye to selectively transmit light of awavelength effective for photoelectric conversion by the photovoltaicpanel. In some implementations, the fluidic coolant is water orrefrigerant. In some implementations, the water or refrigerant includesan anti-foaming agent to inhibit bubbles from being generated in thecooling water. In some implementations, the water or refrigerant furtherincludes an anti-freezing solution to prevent the water or refrigerantfrom being frozen.

In another aspect, electric energy produced using any one of the devicefor photovoltaic power generation or the cooling system for cooling aphotovoltaic panel is provided.

In another aspect, a method of producing electric power using any one ofthe device for photovoltaic power generation or the cooling system forcooling a photovoltaic panel is provided.

Some embodiments of the disclosed technology suggest disposing thephotovoltaic panel in a housing in which a coolant is contained. A lightreceiving surface of the photovoltaic panel is disposed to face atransparent window through which the sunlight is transmitted. Anexemplary example of the coolant is water or a refrigerant. For smoothconvection of this refrigerant, the photovoltaic panel is disposed at anadequate distance from an internal surface of the transparent window,and the distance is set to be small such that a quantity of the sunlightabsorbed by the refrigerant is minimized.

Since the refrigerant having great specific heat is in direct contactwith overall external surfaces (front and rear surfaces) of thephotovoltaic panel, the disclosed technology will have remarkably highercooling efficiency than the cooling technique in the related art.

Some implementations of the disclosed technology allow to reduce thecost of the photovoltaic power generation. That is, when the sameexpense is invested for a photovoltaic power generation plant andmaintenance, the disclosed technology produces more electric powercompared to the related art. The flat mirror is far less expensive thanmeans (a lens, a curved mirror, etc.) required for light condensing inthe related art, is lighter, and can be mass produced.

Some implementations of the disclosed technology allow to preventdeterioration of the photovoltaic panel to extend a service life. Thisis an additional effect that can be obtained from an embodiment in whichthe photovoltaic panel is disposed in a cooling housing.

Some implementations of the disclosed technology can project thesunlight to the photovoltaic panel with a sufficient intensity on acondition that the intensity of the sunlight is low (morning and lateafternoon, winter, high-latitude area, cloudy or foggy weather). Whenthe number of flat mirrors is sufficiently increased, although theintensity of the sunlight is very low, the sunlight can be uniformlycondensed on the photovoltaic panel with an intensity more than theintensity of the sunlight when the sun is located at the zenith.

Some implementations of the disclosed technology can easily carry outthe light condensing on the entire photovoltaic panel with a uniformintensity in spite of low precision. This is an effect that can beobtained by adding a predetermined value to a length and width of eachflat mirror as a margin so as to be greater than values given bym_(x)=p_(x) cos θ_(x) and m_(y)=p_(y) cos θ_(y) to be described below.As a magnitude of the margin increases, required precision may befurther reduced. The reason can be easily understood by those skilled inthe art through the contents of FIGS. 4 and 5.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1a to 1d are representative views of prior patents associated withphotovoltaic power generation using light condensing.

FIG. 1e is a view for describing problems of a conventional condensingtechnique using a lens or a curved mirror.

FIG. 2 is a view for describing a condensing method using flat mirrorsand a rotation mechanism according to the disclosed technology.

FIG. 3 is a view for describing both a cooling method using a housingand the light condensing method using flat mirrors according to thedisclosed technology (representative view of the disclosed technology).

FIGS. 4 and 5 are views for describing a process of calculating lightcondensing capabilities of the flat mirrors and a relationship betweenmagnitudes of the light condensing capabilities.

FIG. 6 is a view for describing the process of calculating the lightcondensing capabilities on the assumption that 24 flat mirrors of FIG. 2are arrayed and have x-y symmetry.

FIG. 7 is a view for describing a cooling housing of the disclosedtechnology in detail.

FIG. 8 is a view for presenting a frame other than that of theembodiment of FIG. 2 and describing the center (Q) of gravity of theframe.

FIG. 9 is a view for presenting a supplementary panel.

FIGS. 10a, 10b and 10c are views for describing calculation of anatmospheric layer passing distance and attenuation of the intensity ofsunlight according to a change in altitude of the sun.

FIG. 10d is a graph illustrating results of calculating a change inlight condensing capability according to a change in distance betweenthe photovoltaic panel and a structure in which the flat mirrors arearranged.

FIGS. 11a and 11b are views for describing a calorific value dischargingcapability of a photovoltaic module immersed in refrigerant.

DETAILED DESCRIPTION

The disclosed technology provides a technique for the photovoltaic powergeneration to reduce the cost of power generation. The governments ofvarious countries including Korea are trying to foster the photovoltaicpower generation industry through a feed-in tariff (FIT) and a renewableportfolio standard (RPS), but they have a limitation due to a financialburden. Thus, the necessity of securing commercially renewablephotovoltaic power generation technology is urgent. This directly meansthat technology for reducing the cost of electric power production in aphotovoltaic power generation field is needed.

It is important to produce more electric power using the photovoltaicpanel having a given area. To this end, an attempt to increasephotoelectric conversion efficiency of a solar cell, an attempt tosuppress a rise in temperature of a photovoltaic module to increasephotovoltaic power generation efficiency, an attempt to increaseproduction of the electric power relative to the cost through lightcondensing, an attempt to produce the photoelectric cell substrate at alow cost, etc. are being made.

Related arts having a deep relation with the technical idea and spiritof the disclosed technology are photoelectric power generationtechnologies to which light condensing and cooling are applied.

FIGS. 1a to 1d are representative views of conventional patenttechniques published in relation to photovoltaic power generationtechnology using light condensing. The representative views ofconventional patent techniques, JP 2009-545186A (FIG. 1a ), JP2003-536244A (FIG. 1b ), JP 2009-533841A (FIG. 1c ), and JP 2009-545877A(FIG. 1d ), are presented in turn. The techniques of FIGS. 1a, 1b and 1cuse lenses or spherical mirrors, and thus still include problems of therelated arts to be described below (see the portion “1. Light condensingusing flat mirrors” of the section “Technical Solution” to be describedbelow). The technique of FIG. 1d is related with the content of thedisclosed technology in that flat mirrors are used for light condensing.However, in the structure illustrated in FIG. 1d , it is difficult toachieve a high level of light condensing which the disclosed technologypursues.

FIG. 1e is a view attached to describe the problems of the related artswhich are pointed out in the portion “1. Light condensing using flatmirrors” of the section “Technical Solution” to be described below.

The related art using a curved lens, a Fresnel lens, or a curved mirrorhas a problem in that a condensed beam has a non-uniform intensity in aradial direction, and is not matched with a rectangular photovoltaicpanel because it has a circular shape.

Further, the condensing technique of FIG. 1b is intended to use a tandemsolar cell that is high in photoelectric conversion efficiency, isexpensive, and has a small area. For this reason, the conventionalcondensing technique is not suitable to condense sunlight on aphotovoltaic panel of a large area with uniform intensity.

Related arts associated with cooling include an air cooling method basedon a heat sink, a water cooling method in which a rear surface of aphotovoltaic module is in contact with a heat sink in which a coolant iscontained or circulated, and so on. However, only the rear surface of aphotovoltaic panel is mainly cooled so as not to disturb incidence ofsunlight, and the coolant is not in direct contact with an externalsurface of the photovoltaic panel. Therefore, cooling efficiency islargely inadequate.

The disclosed technology provides a method and device to which lightcondensing based on a flat mirror, sun tracking for the lightcondensing, and cooling of heat caused by the light condensing areapplied to produce maximum electric power from a photovoltaic panelhaving the same class.

In the disclosed technology, the following three main technical ideasare selectively combined.

1. Light Condensing Using Flat Mirror

Problems of light condensing technology using a condensing lens, aconcave mirror, or a Fresnel lens (hereinafter “conventional lightcondensing devices”) in the related art are as follows.

(1) It is very difficult and expensive to manufacture a photovoltaicsystem using the conventional light condensing devices because very highprecision is required. (2) The shape of a rectangular photovoltaic paneland the shape of the condensed beam (typically in a circular shape) ofthe conventional light condensing devices are mismatched. (3) The weightof the conventional light condensing devices is not small. (4) Theintensity of the condensed sunlight is non-uniform in a radial direction(see FIG. 1e ).

The fact that the intensity of the condensed beam is non-uniform has anadverse effect on the photovoltaic power generation. In the case of asolar collector system distinguished from the photovoltaic powergeneration, although the intensity of the condensed beam is non-uniform,this does not matter. When solar heat is used, a total quantity ofcondensed sunlight reaching a solar collector is only a concern, anduniformity of beams of the condensed sunlight has no particularinfluence on performance. In contrast, in the case of the photovoltaicpower generation, uniformity of beams of the condensed sunlight has avery important influence on performance. This is because output currentsof unit cells connected in series are determined by the smallest valueof values of the output currents of unit cells connected in series, andthe output current of each of the unit cells is proportional to a localintensity of the sunlight incident upon each of the unit cells.

The disclosed technology can easily avoid the problems accompanied withthe light condensing method of the related art at a low cost byoverlapping and condensing uniform reflected light on the photovoltaicpanel using rectangular flat mirrors so as to match a shape (rectangularshape) of the photovoltaic panel.

2. Sun Tracking Method

The disclosed technology employs a photovoltaic power generation methodusing light condensing, and thus may obtain high efficiency when appliedalong with a sun tracking method. A rotation mechanism is a techniquethat has been widely known in the related technical field for a longtime and is commercially available.

3. Cooling

It is reported that a typical silicon photovoltaic module that does notcondense the sunlight rises to a temperature more than 60° C. to 70° C.at noon in the summer, and photoelectric conversion efficiency isreduced by a predetermined rate (by 0.5%) whenever the temperature risesin units of 1°. If the sunlight is condensed on the photovoltaic panelby several times to tens of times by the disclosed technology, a rise intemperature and a rapid drop of the photoelectric conversion efficiencyare inevitable. The prediction of the inventor of the disclosedtechnology is as follows. A portion that is most sensitive to thetemperature rise and has the greatest influence on the photoelectricconversion efficiency is possibly a semiconductor layer. Theconventional cooling method is to cool the outside of a rear surfacethat is relatively more distant from the semiconductor layer. Thismethod does not effectively prevent the temperature rise of thesemiconductor layer that absorbs the sunlight and generates freeelectrons which produce the greatest influence on the photoelectricconversion efficiency.

Various exemplary embodiments of the disclosed technology will bedescribed in detail with reference to the attached drawings.

FIG. 2 is a view presented to be able to intuitively understand uniformlight condensing based on flat mirrors and a sun tracking method for thelight condensing, and FIG. 3 illustrates a method of cooling heat causedby the light condensing.

A description made with reference to FIG. 2 is as follows.

A photovoltaic panel is disposed with a light receiving surface thereofagainst the sun in a direction perpendicular to an incident direction ofsunlight. That is, when a traveling direction of the sunlight is definedas the +z axis, a normal to the light receiving surface of thephotovoltaic panel is a direction of the +z axis. Here, it should benoted that, for the convenience of calculation, it is best and efficientto dispose the photovoltaic panel in such a manner that, when a unitvector in the traveling direction of the sunlight is (0, 0, 1), a unitvector in a direction normal to the light receiving surface is (0, 0,1). However, to prevent an attempt to avoid the scope of the claims ofthe disclosed technology, the unit vector in the direction normal to thelight receiving surface of the photovoltaic panel is not limited to (0,0, 1) as described above.

A plurality of flat mirrors are each fixed with a reflective surfacefacing the sun at adequately inclined angles in the directions of the xand y axes, and reflect incident sunlight to the photovoltaic panel asreflected light having a uniform intensity. The flat mirrors are eachset to be different in size and direction according to a position, butthey should be set to such an extent that the reflected light thereoffully covers the light receiving surface of the photovoltaic panel.

An additional description will be made to give grounds for defining theclaims of the disclosed technology. An important aspect of the disclosedtechnology is that the photovoltaic panel is disposed with the lightreceiving surface thereof against the sun. This may be expressed asfollows: when the unit vector in the traveling direction of the sunlightis (0, 0, 1), a z-axial component of the unit vector in a directionnormal to the light receiving surface of the photovoltaic panel ispositive. If not so (i.e. if the z-axial component of the unit vector ina direction normal to the light receiving surface of the photovoltaicpanel is negative or zero, this means that the light receiving surfaceof the photovoltaic panel faces the sun or is parallel with thetraveling direction of the sunlight), it is very difficult to raiselight condensing of a uniform light intensity using the flat mirrors toa high level. This may be easily understood using plane geometry, and sodetailed description thereof will be omitted.

With use of the condensing technique of the disclosed technology, it ispossible to easily project the sunlight condensed to a uniform highlevel onto the photovoltaic panel having a large area. A degree of thelight condensing may be easily adjusted by changing the number of flatmirrors. The degree of the light condensing (intensity of incidentlight) may be changed using, for instance, a method of sufficientlyinstalling the number of flat mirrors in preparation for the winterseason (or in morning and evening) and covering some of the flat mirrorswith an opaque object or changing directions thereof when the intensityof the sunlight is too high in the summer season (or at noon). To thisend, a separate means capable of changing the number of flat mirrorsused in the light condensing may be further provided.

A frame fixes the photovoltaic panel and the plurality of flat mirrorssuch that a predetermined relationship is maintained with respect to thetraveling direction of the sunlight. The frame is also connected to arotation mechanism that rotates in directions of two axes of ahorizontal and vertical axis.

The rotation mechanism rotates the frame in the directions of two axesof a horizontal and vertical axis according to a change in position ofthe sun. Thereby, a predetermined relationship between the lightcondensing of the photovoltaic panel and the flat mirrors fixed to theframe is maintained.

A supporting structure supports weights of the constituent members (theframe, the rotation mechanism, the photovoltaic panel, and the flatmirrors), and mainly it is fixed to the ground, and may be designed tobe movable.

An embodiment of a cooling method will be described with reference toFIG. 3.

When 24 flat mirrors are used as in the embodiment of FIG. 2, theintensity of the sunlight incident upon the photovoltaic panel may beincreased 20 times more than that of a typical case. A temperature ofthe photovoltaic panel is rapidly raised by this strong incident lightParticularly, crystalline silicon solar cells lose their function at ahigh temperature. Thus, in the case of at least the crystalline siliconsolar cells, cooling is inevitably required.

As illustrated in FIG. 3, the most effective method is to dispose thephotovoltaic panel in a housing having a transparent window capable oftransmitting the sunlight and fill the remaining space inside thehousing with a coolant (typically, water). It is more effective to forcethe coolant to flow into/out of the housing.

It may be considered that a circulator (FIGS. 7 and 9) is provided suchthat the coolant circulates in the housing even when the coolant doesnot flow into/out of the housing. Even if there is not theinflow/outflow and forcible circulation type of the coolant, when themass of the coolant is sufficient, at least external surface of thephotovoltaic panel may be cooled within the range of a fixed temperatureby thermal convection of the coolant itself.

A predetermined relationship between the photovoltaic panel and the flatmirrors, a size, and an intensity of reflected light will be describedin greater detail with reference to FIGS. 4, 5 and 6.

In FIG. 4, it is assumed that the traveling direction of the sunlight isthe direction of the +z axis. It is assumed that the light receivingsurface of the photovoltaic panel is directed in the direction of the +zaxis (i.e., the light receiving surface of the photovoltaic panel isbacked by the sun), and a center P of the light receiving surface islocated at a point (0, 0, 0).

When a reflective surface of the flat mirror is directed in thedirection of the +z axis, a center M of the reflective surface islocated at a point (0, d, l), and an angle formed between a line segmentPM and the z axis is 2θ, if the reflected light caused by the flatmirror covers the entire photovoltaic panel, the flat mirror shouldrotate about a straight line (y=d, z=l) parallel to the x axis by θ andbe inclined. Hereinafter, this rotation is referred to as “θ_(y)”regardless of a direction of a center axis of the rotation. Likewise, anangle of the flat mirror that should be inclined according to x-axialseparation between the center of the light receiving surface of thephotovoltaic panel and the center of the reflective surface of the flatmirror is referred to as “θ_(x)”.

Arranging the relationships at this time, relationships of tan 2θ=d/l,cos θ=m/p, and cos θ=b/m are formed (where p is the y-axial width of thephotovoltaic panel, m is the y-axial width of the flat mirror, and b isthe y-axial width of the sunlight that is reflected by the flat mirrorand is incident upon the photovoltaic panel).

Thus, to fully cover the photovoltaic panel in which one side thereofhas a length p with the reflected light, a length of the correspondingside of the flat mirror has a minimum value of m=p cos θ. At that time,a width of the corresponding side of the incident sunlight is b=p cos²θ.

The above relationships are equally applied to the flat mirror separatedin the direction of the x axis.

Therefore, in the case of the reflected light of the flat mirrorinclined by θ_(x) in the direction of the x axis and θ_(y) in thedirection of the y axis, the sunlight incident upon an area ofb_(x)×b_(y)=p_(x) cos² θ_(x)×p_(y) cos² θ_(y) is uniformly reflected tothe photovoltaic panel having an area of p_(x)×p_(y), a light condensingcapability(g) of the flat mirror inclined by (θ_(x), θ_(y)) is given asfollows.

Light condensing capability(g) of a flat mirror inclined by (θ_(x),θ_(y)): cos² θ_(x)×cos² θ_(y)

The light condensing capability will be described below by way of afurther embodied example.

When the center M of the reflective surface of a certain flat mirror islocated at a point (d_(x), d_(y), l), and the center P of the lightreceiving surface of the photovoltaic panel is located at the point (0,0, 0), the angle formed between the line segment PM and the z axis is2θ_(y) when viewed in the y-z plane, and 2θ_(x) when viewed in the x-zplane. The following relationships are formed between them.

tan 2θ_(y) =d _(y) /l, cos θ_(y) =m _(y) /p _(y), and cos θ_(y) =b _(y)/m _(y)

tan 2θ_(x) =d _(x) /l, cos θ_(x) =m _(x) /p _(x), and cos θ_(x) =b _(x)/m _(x)

In the above numerical relationships, when the size (width×length) ofthe photovoltaic panel is p_(x)×p_(y), a necessary minimum size(width×length) of the flat mirror is m_(x)×m_(y)=p_(x) cos θ_(x)×p_(y)cos θ_(y). Here, the sunlight that is reflected by the flat mirror andis uniformly incident upon the photovoltaic panel means that a quantityof light incident upon the rectangular plane b_(x)×b_(y)=p_(x) cos²θ_(x)×p_(y) cos² θ_(y) perpendicular to the original traveling directionof the sunlight prior to reflection is uniformly projected to the areap_(x)×p_(y) of the photovoltaic panel.

Thus, the light condensing capability of the flat mirror inclined by(θ_(x), θ_(y)) in this way is cos² θ×cos² θ_(y).

Let's look at how much the 24 flat mirrors of FIG. 2 condense thequantity of the sunlight with reference to FIGS. 5 and 6. For theconvenience of calculation, it is assumed that p_(x)=p_(y)=p, i.e. thephotovoltaic panel has a square shape, and the flat mirrors aresymmetrically arrayed in the x-y plane. Then, as in FIG. 6, the 24 flatmirrors will have five light condensing capabilities of G1 to G5.

In FIG. 5, when the setting conditions are d1=1.2p, d2=2.4p, and L=3p,5p, and 0.5p, each light condensing capability will be calculated asfollows.

When L=5p,

2θ_(x1)=2θ_(y1)=2θ₁=arctan(d1/L)=actan(1.2/5)=13.50°, and thus θ₁=6.75°

2θ_(x2)=2θ_(y2)=2θ₂=arctan(d2/L)=actan(2.4/5)=25.64°, and thus θ₂=12.82°

Light condensing capability of G1: cos² θ₁×1=0.986×1=0.986 (four), andthus one of θ_(x1) and θ_(y1) is 0°

Light condensing capability of G2: cos²θ₂×1=0.951×1=0.951 (four), andthus one of θ_(x2) and θ_(y2) is 0°

Light condensing capability of G3: cos² θ₁×cos² θ₁=0.986×0.986=0.972(four)

Light condensing capability of G4: cos² θ₁×cos² θ₂=0.986×0.951=0.938(eight)

Light condensing capability of G5: cos² θ₁×cos² θ₂=0.951×0.951=0.904(four)

When L=5p, the 24 flat mirrors condense the incident light by 22.756times (under the condition of the symmetrical array).

When L=3p,

2θ_(x1)=2θ_(y1)=2θ₁=arctan(d1/L)=actan(1.2/3)=21.80°, and thus θ₁=10.90°

2θ_(x2)=2θ_(y2)=2θ₂=actan(d2/L)=actan(2.4/3)=38.66°, and thus θ₂=1933°

Light condensing capability of G1: cos²θ₁×1=0.9641=0.964 (four), andthus one of θ_(x1) and θ_(y1) is 0°

Light condensing capability of G2: cos²θ₂×1=0.8901=0.890 (four), andthus one of θ_(x2) and θ_(y2) is 0°

Light condensing capability of G3: cos²θ₁×cos²θ₁=0.964×0.964=0.929(four)

Light condensing capability of G4: cos²θ₁×cos²θ₂=0.964×0.890=0.858(eight)

Light condensing capability of G5: cos²θ₂×cos²θ₂=0.890×0.890=0.792(four)

When L=3p, the 24 flat mirrors condense the incident light by 21.164times (under the condition of the symmetrical array).

When L=0.5p,

2θ_(x1)=2θ_(y1)=2θ₁=arctan(d1/L)=actan(1.2/0.5)=6738°, and thusθ₁=33.69°

2θ_(x2)=2θ_(y2)=2θ₂=arctan(d2/L)=arctan(2.410.5)=78.23°, and thusθ₂=39.12°

Light condensing capability of G1: cos² θ₁×1=0.6921=0.692 (four), andthus one of θ_(x1) and θ_(y1) is 0°

Light condensing capability of G2: cos²θ₂×1=0.6021=0.602 (four), andthus one of θ_(x2) and θ_(y2) is 0°

Light condensing capability of G3: cos²θ₁×cos²θ₁=0.692×0.692=0.479(four)

Light condensing capability of G4: cos²θ₁×cos²θ₂=0.692×0.602=0.417(eight)

Light condensing capability of G5: cos²θ₂×cos²θ₂=0.602×0.602=0.362(four)

When L=0.5p, the 24 flat mirrors condense the incident light by 11.876times (under the condition of the symmetrical array).

That is, it can be found that, despite of the fact that a distance Lbetween the photovoltaic panel and the frame plane in which the flatmirrors are arrayed is a near distance of 0.5p, the 24 flat mirrors ofFIG. 2 condense the sunlight by at least 10 times.

This is as follows in principle. No matter how small L is, under thecondition that all the reflective surfaces of the flat mirrors arelocated on a right side (z>0) of the plane that is z=0 (i.e. the planein which the light receiving surface of the photovoltaic panel isinfinitely stretched, that is, the x-y plane), θ₁ and θ₂ are smallerthan 45°. Thus, the light condensing capabilities of G1 to G5 under thecondition of the array of the 24 flat mirrors and the x-y symmetricalarray in FIG. 6 are greater than 0.5 (four), 0.5 (four), 0.25 (four),0.25 (eight), and 0.25 (four) respectively, and the total lightcondensing capability is greater than 8, provided that the analogy ispossible under the assumption that the flat mirrors may be arrayed so asnot to mutually disturb paths of the incident light and the reflectedlight. If a magnitude of L is too small, it is difficult to array theflat mirrors such that no obstacle is located on the optical paths.

Let's substitute the case of L=0.25p. When L=0.25p and the otherconditions (magnitudes of d1 and d2) are the same as the above examples,θ₁=39.16°, θ₂=42.03°, the light condensing capabilities of G1 to G5 are0.601, 0.552, 0.361, 0332, and 0.305 respectively, and the total lightcondensing capability of the 24 flat mirrors becomes 9.932.

When L has an infinite magnitude (both θ₁ and θ₂ converge on 0°), allthe light condensing capabilities of G1 to G5 become 1, and the totallight condensing capability caused by the 24 flat mirrors of FIGS. 2 and6 becomes 24.

However, it is very inefficient to greatly set L in order to increasethe total light condensing capability. This is because, when themagnitude of L exceeds 5p (where 5p is approximately similar to a lengthand a width of a reflector structure which the 24 flat mirrors of FIG. 2constitute), a volume (strictly speaking, a z-axial length) of astructure of the photovoltaic power generation system is increased,whereas the total light condensing capability is not particularlyincreased. It is easier and more effective to reduce the distance L upto a plane formed by the centers of the reflective surfaces of the flatmirrors as small as possible and increase the number of flat mirrors inreturn. (Here, the centers of the reflective surfaces of the flatmirrors may be approximately considered to be present on the same plane.However, this does not mean that the reflective surfaces of the flatmirrors should be strictly present on the same plane.)

The number of flat mirrors may be easily increased by further disposinganother 24 flat mirrors around the flat mirrors of FIG. 2. That is, inFIG. 2, flat mirrors surround the flat mirrors in duplicate (8+16). Ifnecessary, 24 new flat mirrors may be further disposed along a thirdcircumferential line.

Moreover, all the flat mirrors always project uniform reflected light tothe light receiving surface of the photovoltaic panel, and thusuniformly condensed sunlight is always projected to the light receivingsurface of the photovoltaic panel regandless of a number and disposedpositions of the flat mirrors that are used.

Under the above conditions (the square photovoltaic panel (p×p),d1=1.2p, and d2=2.4p), when L=αp and 0.1≦α≦10, the light condensingcapability of the 24 flat mirrors of FIG. 6 is calculated and given inTable 1 below and a graph of FIG. 10 d.

Here, it has already been described that there are the followingrelationships:

G1=cos²θ₁, G2 cos²θ₂, G3=cos⁴θ₁, G4=cos² θ₁×cos²θ₂, and G5=cos⁴θ₂, and

2θ₁=arctan(d1/L) and 2θ₂=actan(d2/L)

TABLE 1 Light condensing capability = α θ₁ (radian) θ₂ (radian) 4(G1 +G2 + G3 + G5) + 8G4 0.1 0.743827547 0.764576874 8.763600258 0.20.702823825 0.743827547 9.545895567 0.3 0.662908832 0.72322066610.33379599 0.4 0.624522886 0.702823825 11.1147626 0.5 0.5880026040.682700469 11.87783205 0.6 0.553574359 0.662908832 12.61424931 0.70.521360939 0.643501109 13.31767994 0.8 0.491396862 0.62452288613.98408329 0.9 0.463647609 0.606012828 14.61137645 1 0.4380290250.588002604 15.1990133 2 0.27020975 0.438029025 19.22910132 30.190253189 0.337370471 21.17793504 4 0.145728397 0.27020975 22.18729915 0.11777249 0.223759988 22.75500219 6 0.09869778 0.19025318923.09884423 7 0.084889137 0.165148677 23.32036492 8 0.0744449740.145728397 23.4704606 9 0.066275766 0.130301196 23.57643781 100.059714463 0.11777249 23.65384798

Anyone can understand that it is reasonable and realistic to set awithin a range between 0.5 and 5 from the graph of FIG. 10 d.

It has already described that it is not necessary for all the reflectivesurfaces of the flat mirrors to be present on the same plane. Rather, itis better for the reflective surfaces of the flat mirrors to bealternately disposed in two or more virtual planes that are propertyseparated. This is because a wind smoothly flows into an empty spacebetween the flat mirrors according this configuration, and the flatmirrors and the frame may better withstand a strong wind. The reason thesufficient distances such as d1=1.2p and d2=2.4p are set between theflat mirrors in the previous embodiment is to consider that an influencecaused by the wind should be reduced. (In fact, a mom important reasonthan this is that the flat mirrors should be set to have a margin to asufficient extent more than a necessary minimum width in order toovercome an array error as will be described below.)

It is good that a width m of one side of the flat mirror is slightlygreater than a required minimum magnitude of p cos θ. This is becausethe reflected light can always fully cover the light receiving surfaceof the photovoltaic panel in spite of an array error of the flatmirrors, an array error of the photovoltaic panel, an operation error ofthe rotation mechanism, and so on.

The sunlight condensing using the flat mirrors has been sufficientlydescribed hereby.

Hereinafter, the cooling method will be described in greater detail.

As described above, with use of the 24 flat mirrors of FIG. 2, thesunlight may be condensed by 8 to 24 times according to the z-axialdistance L between the photovoltaic panel and the flat mirrors (it ispractically reasonable to condense the sunlight by 10 to 22 times). Inthis case, particularly in the crystalline silicon solar cells, it isimpossible to normally produce the electric power without cooling due toa rise in temperature caused by the light condensing.

It is a well-known phenomenon that power production efficiency increaseswhen the photovoltaic panel is cooled. It is reported that, even whenlight is not condensed, a temperature of crystalline siliconphotovoltaic modules increases up to 60° C. to 70° C. in a fine weatherin the middle of summer, photoelectric conversion efficiency is loweredby a certain rate (approximately by 0.5%) whenever the temperatureincrease in units of 1° C., and a function is stopped when thetemperature is more than 100° C. When the incident sunlight is condensedby more than 10 times as in the embodiment of the disclosed technology,it is essential to cool the photovoltaic panel, and particularly thecrystalline silicon photovoltaic panel, and it is necessary tosignificantly increase cooling efficiency although a cost is slightlyincreased.

Another aspect of the disclosed technology is based on the above fact.

An exemplary embodiment of the cooling method of the disclosedtechnology will be described with reference to FIG. 7.

The photovoltaic panel is removably installed in a sealed housing by aholder. A transparent window is provided to be removable from thehousing in front of the light receiving surface of the photovoltaicpanel such that the sunlight reflected by the flat mirrors may betransmitted (the reason the photovoltaic panel and the window areremovably designed is that maintenance such as replacement or repair ofparts, removal of impurities such as scale or a precipitate, and so onis convenient).

The housing is coupled with the window, and thereby the interior thereofis sealed.

The housing is provided with inlet and outlet valves for inflow andoutflow of a coolant (for example, water). Additional holes may also bedrilled in the housing so as to guide two or more power cables connectedto the photovoltaic panel and various electric wires provided for acoolant circulator, and various sensors such as a temperature sensorand/or for a controller which may be added in the housing. It is naturalthat the housing is entirely sealed such that the coolant does not leakout.

The reason the water is used as the coolant is that the water isinexpensive, can be easily obtained, is present in quantity, is notharmful to an environment, and is one of materials having greatestspecific heat. What the great specific heat means is that an effect ofinhibiting a rise in temperature of the photovoltaic panel is great (aconventional air cooling method is largely inadequate in coolingefficiency). In the embodiment of the disclosed technology, since thewater acting as the coolant is in direct contact with both surfaces ofthe photovoltaic panel, the cooling efficiency is remarkably raisedcompared to the conventional water cooling method of cooling only therear surface of the photovoltaic panel.

Of course, close attention should be paid to insulation since thephotovoltaic panel is immersed in water, and an additional cost will berequired. However, a current technical level can sufficiently overcomethis drawback. Although the cost added to this is considered, thecooling method of the disclosed technology will have a sufficienteconomical benefit.

All of the photovoltaic panel, terminals, and electric wires aresurrounded with insulators. Particularly, the insulator covering thelight receiving surface should have excellent permeability (especiallywith respect to light having a wavelength band contributing tophotoelectric conversion). The insulators must have an excellentdielectric characteristic, and preferably have as high a heat transferefficiency as possible. The insulators are preferably as thin aspossible to increase the heat transfer efficiency, but they should havethicknesses such as not to deteriorate the dielectric characteristic anddurability.

Internal surfaces of the window and the light receiving surface of thephotovoltaic panel are disposed as closely as possible. This is intendedto reduce a quantity of the sunlight absorbed by the coolant or therefrigerant). However, avoid disposing the internal surfaces of thewindow and the light receiving surface of the photovoltaic panel soclosely that it is hard for the coolant or the refrigerant adjacent tothe light receiving surface of the photovoltaic panel to causeconvection.

A replacement period of the refrigerant will be calculated on theassumption that the coolant is water. When the area of the lightreceiving surface of the photovoltaic panel is A, an area of the windowof the housing should be greater than A. It is firstly assumed that across-sectional area of the housing including the window is A. When aninternal thickness of the housing is D, a volume of the refrigerantcontained in the housing is AD. A volume occupied by, for instance, thephotovoltaic panel may be ignored. Assume that it takes time t for thetemperature of the coolant to rise from 20° C. to 50° C. (ΔT=30K), thefollowing expression is formed.

ΔQ=ADρσΔT=gI ₀ At

The above expression means that the quantity of energy required to raisethe temperature of the water in the housing by 30° C. is equal to thequantity of energy introduced by the reflected light incident upon thephotovoltaic panel.

Here, ρ=1 g/cm³ (the density of water), σ=42 J/gK (the specific heat ofwater), gI₀ indicates the intensity of the reflected light incident uponthe photovoltaic panel, and g indicates the light condensing capabilitycaused by the 24 flat mirrors of FIG. 2 and its magnitude has a valuebetween 8 and 24 (practically between 10 and 22) as described above. 1 ois the intensity of sunlight, and applying the greatest value on theEarth's surface of AM1=925 W/m² (intensity of sunlight radiating fromthe sun at the zenith on a fine day) is sufficient (the above conditionis the worst condition in regard to the temperature rise).

Arranging the above expression by substituting the above numericalvalues for the above expression, a relationship of t=(D/g)(1362)(sec/cm)[sec] is formed.

It can be seen from FIG. 10d that the worst practical situation (fromthe aspect of the temperature rise) is roughly when g=22. Substitutingthis value, t=(D)(61.9)(sec/cm) [sec].

The above expression means that, when the internal width D of thehousing is 10 cm, it takes 619 seconds (more than 10 minutes) to raisethe temperature by 30° C. When D is designed to be 60 cm, this meansthat the temperature of the refrigerant may be maintained within amaximum setting value (50° C.) without replacing the refrigerant for atleast one hour.

In consideration of the weight of the housing, maintenance expenses, andso on, it can be easily selected by a user whether to design D to besmall and to frequently replace the refrigerant or whether to design Dto be great and to rarely replace the refrigerant. It is not difficultto maintain the temperature of the refrigerant in the housing within avery small fixed range by forcing a proper quantity of refrigerant toflow at proper periods or by installing a sensor to control acirculating rate of the refrigerant.

It has previously been described that the housing is further providedwith the inlet and outlet valves for inflow and outflow of the coolant.Further, it has previously been mentioned that the circulator forforcibly circulating the coolant in the housing may be further provided.The circulator may be implemented by a rotating fan or a jet device. Thecirculator is useful to prevent the temperature of the refrigerantaround the light receiving surface of the photovoltaic panel from risinghigher than the temperature of the refrigerant at the other portions.However, the circulator is not essentially required, and may be replacedwith heat distribution caused by the thermal convection of therefrigerant.

Next, it will be calculated whether the photovoltaic panel immersed inthe refrigerant discharges a calorific value, which corresponds to acalorific value obtained by receiving the condensed sunlight, to thecoolant. FIG. 11 illustrates a general structure of the photovoltaicmodule using crystalline silicon.

Assuming that thermal conductivity of a sealing material is k1, athickness of the sealing member up to a front transparent panel is d1,thermal conductivity of the front transparent panel is k2, and athickness of the front transparent panel is d2, thermal resistance ofthe sealing material toward a front surface is R1=d1/k1, and thermalresistance of the front transparent panel is R2=d2/k2. Therefore, thetotal thermal resistance of a path ranging from a solar cell element tothe refrigerant located at the front surface is R=R1+R2=d1/k2+d2/k2, anda transferred calorific value is obtained as Q=A(ΔT)t/R (where ΔT is thetemperature difference between the refrigerant and a surface of thesolar cell element, and t is the time for which the calorific value isdischarged).

Assuming that d1=2 mm, d2=5 mm, k1=1 W/mK (resin of a type having veryhigh thermal conductivity), k2=0.8 W/mK (general glass), and ΔT=1K(assuming that the temperature of the solar cell element is 60° C., andthe temperature of the refrigerant is 50° C.), the total thermalresistance is R=8.25×10⁻³ (m²K/W), (ΔQ)/tA=(ΔT)/R=1,212 W/m². That is,this means that the calorific value may be discharged toward the frontsurface with power of 1,212 W per 1 m². Since the side of the rearsurface of the photovoltaic module does not need to be transparent, thethermal resistance toward the rear surface may be designed to be evensmaller than that toward the front surface (for example, the rear panelmay be formed of a metal plate having even higher thermal conductivitythan plate glass most frequently used as the front transparent panel).

Thus, it can be found that a heat discharging capability is at leastmore than 2×1,2122,424 W/m² by means of the embodiment of FIG. 11a anddesignation of the variables (d1, d2, k1, k2, ΔT) in consideration ofboth the front and rear surface sides. The total thermal resistance Rmay be made smaller by reducing d1 and d2 and increasing k1 and k2(i.e., the heat discharging capability toward the refrigerant may bemade greater).

Further, when ΔT is great due to a low temperature of the refrigerant,that much more calorific value is better discharged.

FIG. 11b illustrates a general structure of a thin film photovoltaicmodule (formed of amorphous silicon, CdTe, CIGS, etc.). In comparisonwith the case of FIG. 11a , the total thermal resistance may be furtherreduced since the transparent resin located at the front surface is notrequired. Unlike the case of FIG. 11a , in the case of FIG. 11b , sincea sealing material (generally a polymeric resin) may be opaque, the factthat a width of selection for greatly setting thermal conductivity k iswider also has a favorable effect.

Meanwhile, since a polymeric resin in which the thermal conductivity kreaches 7 W/mK is commercially available, it is not difficult to reducethe total thermal resistance compared to the assumption of the aboveembodiment in terms of the current technical level.

It has been sufficiently considered that the photovoltaic moduleimmersed in the refrigerant may discharge the calorific value equivalentto the calorific value received from the condensed sunlight to therefrigerant.

To minimize absorption of the sunlight caused by the refrigerant, thelight receiving surface of the photovoltaic panel and the internalsurface of the window are disposed as close as possible. However, thedistance between the two surfaces should not be so close as to hinderthe convection of the refrigerant because the accumulated thermal energyin the vicinity of the light receiving surface of the photovoltaic panelshould be dissipated as soon as possible. This is because the convectionof the refrigerant whose temperature rises in the vicinity of the lightreceiving surface of the photovoltaic panel must be sufficiently allowedto take place.

A small quantity of anti-foaming agent for inhibiting generation of airbubbles may be added to the refrigerant. This is because, when the airbubbles floating in the refrigerant are increased, the path of thesunlight incident upon the photovoltaic panel is disturbed.

When a temperature is below zero, an anti-freezing solution may be mixedwith the refrigerant at a certain ratio. However, considering problemssuch as an adverse effect on the environment, an increase in corrosionof the photovoltaic power generation system, addition expenses requiredfor purchase and management of the anti-freezing solution, and so on,pure water is most preferably used as the refrigerant as far aspossible. The refrigerant is seldom frozen during the daytime (theperiod for which the electric power is produced) on a fine day on whichthe sun shines due to heat caused by the light condensing of the flatmirrors. A problem occurring when the refrigerant is frozen is that thepath of the sunlight incident upon the photovoltaic panel is disturbedby ice in the morning when the temperature is below zero, and the powerproduction efficiency is reduced. The more serious problem is that apressure increases inside the housing, the valves and/or hoses due tovolume expansion of the coolant (water) occurring in freezing can causemechanical breakdown of the constituents. This problem mainly takesplace for a time when the power production is stopped (the operation isstopped), i.e. when the sun does not shine (at night or on a cloudyday).

However, even when the refrigerant is the pure water, it is difficult tosay that no method of preventing the mechanical breakdown is present. Ifthe refrigerant inside the systems such as the housing, the valves, andinflow/outflow pipes for the coolant is completely drained at night whenthe temperature is below zero or on a sub-zero rainy day when there islittle possibility of the sun shining, and the refrigerant is filledimmediately before the sun begins to shine to start power generation,the above-mentioned problems such as the problem that the refrigerant isfrozen to delay operation for the photovoltaic power generation and themechanical breakdown caused by the volume expansion may be solved. Thinice covered on the window will quickly be melted due to the heat causedby the light condensing.

In addition, since the photovoltaic panel is installed in and protectedby the housing as in FIG. 7, the photovoltaic panel may be made thinner(within a range in which mechanical strength is allowed), anddeterioration (mainly attributed to exposure to short wavelengthradiation such as ultraviolet and temperature variation) occurring whenthe photovoltaic panel is exposed to the air in the past may be reduced.

Another embodiment associated with FIG. 2 will be described withreference to FIG. 8.

In FIG. 8, one photovoltaic panel and 24 flat mirrors are arrayed ineach of four subframes illustrated. In this case, unlike thoseillustrated in FIG. 2, a rotation mechanism is most preferably connectedto a center Q of gravity of the subframes. The reason is that, when therotation mechanism is connected to the center Q of gravity of all of thesubframes, a driving force required to rotate the subframes isminimized, and a mechanical fatigue caused by gravity or wind may beminimized. Even in the case of FIG. 2, it is natural that the center ofgravity of the frame is more preferably found out and connected to therotation mechanism

Further, it is natural that shapes of the subframes should be designedso as not to disturb the path of the sunlight.

As illustrated in FIG. 9, the rear surface of the photovoltaic panel (orthe cooling housing) receiving the sunlight that is reflected andcondensed by the flat mirrors may be further provided with a separatesupplementary panel. This is because the previously provided rotationmechanism may be used, and the sunlight incident upon the photovoltaicpanel or the rear surface of the housing are uselessly wasted andinstead causes only a rise in temperature (in the case of the aboveembodiment, as much energy is dissipated to the outside in the form ofelectric power as a quantity that at least corresponds to conversionefficiency of the supplementary panel).

Although not illustrated, an optical filter layer may be provided on thesurface(s) of the window and/or the flat mirrors to filter light of awavelength band that does not contribute to the photoelectricconversion. This has an additional effect of inhibiting the temperaturerise and the deterioration of, for instance, the photovoltaic panel.Such a band pass optical filter is most preferably attached to the flatmirrors, because the maximum value of the intensity of the sunlightreceived by the flat mirrors is AM1 (925 W/m²), and thus deteriorationof the optical filter caused by heat or strong light may be minimized.In this case, however, an amount of required optical filters increases,which is unfavorable in terms of a price. This is opposed to the case inwhich the optical filter is attached to the window. The optical filtermay be attached to the light receiving surface of the photovoltaicpanel. However, this is the worst method, because this means that acalorific value is accumulated on the object (optical filter) that is indirect contact with the light receiving surface of the photovoltaicpanel. Further, a material (e.g. a dye) performing a function similar tothat of the optical filter may be mixed with the coolant in the housing.Use of the optical filter may be selected independently or incombination of two or more of the four cases (the flat mirror, thewindow, the photovoltaic panel, and the coolant) according to itsposition.

The flat mirrors should have high reflectance, low transmittance, andlow absorptance with respect to the incident light (particularly, theincident light of the wavelength band contributing to the photoelectricconversion), and the window should have low reflectance, hightransmittance, and low absorptance.

The following will be added to the technical idea of the disclosedtechnology.

1. The embodiment of the disclosed technology is described keeping inmind the crystalline silicon solar cells that are commercially sold.However, it is apparent that the embodiment of the disclosed technologymay also be applied to other photovoltaic panels made up of thin filmsolar cells formed of amorphous silicon, CIS, CIGS, and CdTe, compoundmulti-junction solar cells formed of a III-V group or II-VI groupelement, solar cells having a quantum structure, dye-sensitized solarcells, organic thin film solar cells, Plasmon solar cells, or the like.

2. The cooling system of the disclosed technology is expected to beessentially required for silicon solar cells. However, when such aphotovoltaic panel in which the photoelectric conversion efficiency isnot particularly reduced in spite of the temperature rise, the coolingsystem presented in the embodiment of the prevent invention is notessentially required.

3. The photovoltaic power generation method using the light condensingand the rotation mechanism of the disclosed technology is based on thegeneral principle of the photoelectric effect in which the producedelectric current (i.e., electric power) is proportional to the quantity(or the intensity) of incident light and which was discovered byEinstein (since an output voltage is primarily determined in proportionto the number of series connections of unit cells in the photovoltaicpanel regardless of the intensity of the incident sunlight, an outputvoltage and an output current which are output from the photovoltaicpanel are in direct proportion). However, when the quantity (intensity)of the incident light continues to increase, the photovoltaic panel doesnot continue to continuously increase a value of the output current(electric power). It is probably predicted that a phenomenon that, whenthe quantity (intensity) of the incident light is equal to or more thana predetermined value, although the quantity (intensity) of the incidentlight continues to increase, the output current (electric power) of thephotovoltaic panel converges to a predetermined value (saturated), or arate of increment dp/di is reduced occurs (where “p” is the electricpower, and “ ” is the intensity of light). However, an aspect of a valueof saturation of the electric current (electric power) or a rate ofchange (dp/di) of the output current according to the intensity of lightwill depend on a type of a semiconductor material constituting thephotovoltaic panel, a thickness of a semiconductor layer, a dopingconcentration, a thickness of a depletion layer, a stacked structure ora method of manufacturing the stacked structure. The simplest method isto increase the thickness of the semiconductor layer (an active layer ora functional layer). The expression “ . . . incident radiation may beabsorbed by functional layers. At this time, a thickness of eachfunctional layer determines a ratio of radiated power absorbed in thefunctional layers, . . . ” is given in Korean Unexamined PatentApplication Publication No. 10-2007-0004928 A (equivalent to WO2005-096394 A1), lines 10 to 12. In this way, it is disclosed that, asthe thickness of the semiconductor layer (functional layer) increases,more light is absorbed, and more electric current is generated. Asdescribed above, the prediction of the present inventor that the aspectof the value of saturation of the electric power or the rate of change(dp/di) of the output current is associated with the thickness of thesemiconductor layer is supported.

4. The technique adapted to be able to increase the value of saturationof the electric current (electric power) of the photovoltaic panel tomaintain the value of dp/di to be high according to the increase of theaforementioned intensity i of light will be a meaningful and usefulinvention by itself. However, if such an invention is made, thisinvention obtains a motive of the invention from the technical ideapublished by the disclosed technology, and thus should be recognized tobe an improved invention (use invention) in relation with the disclosedtechnology.

5. As described in item 3, as the intensity of light increases, theelectric power produced by the photovoltaic panel will be finallysaturated or the rate of increment will slow. However, in the case of atleast the silicon photovoltaic panel that is commercially available, thevalue of dp/di has a fixed value until the intensity of light at leastreaches a magnitude of 1,000 W/m². This point definitely supports theutility of the disclosed technology. This is because, if the disclosedtechnology is used even when the intensity of the sunlight is weak (whenan altitude of the sun is low, the morning and the late afternoon of theday, the winter, a high-latitude area, a cloudy or foggy weather, etc.),the sunlight may always be projected onto the light receiving surface ofthe photovoltaic panel with the intensity of 1 KW/m² (this value isslightly higher than AM1; 925 W/m²) that is the intensity of thesunlight when the sun is at the zenith, i.e. when the sunlight isvertically projected to the Earth's surface). That is, when thephotovoltaic panel having the same area is used, although the number offlat mirrors and the land area required for the photovoltaic powergeneration plant are increased, the electric power may be produced evenin an existing environment in which the intensity of the sunlight isweak and the production of the electric power was given up at the timeof the disclosed technology being applied. This means that thephotovoltaic power generation project is possible in Mongolia, Siberia,Canada, etc. where affordable lands that may be used for thephotovoltaic power generation are available but are short of an amountof solar radiation of the sunlight due to high latitudes, and areestimated that the photovoltaic power generation project is not acompetitive project.

6. Although not greatly relevant to the scope of the claims of thedisclosed technology, it is predicted using a very simplified model howthe intensity of the sunlight is changed according to a change inaltitude of the sun (see FIGS. 10a, 10b, and 10c ).

As illustrated in FIG. 10a , the reason the intensity of the sunlight isreduced in proportion to the altitude of the sun is that the sunlight isblocked (absorbed, reflected, or scattered) by particles such asmolecules or dust in the air. The more blocking particles (gas andliquid molecules, solid particles) present in the traveling path of thesunlight, the lower the intensity of the sunlight. This means that, asthe traveling path of the sunlight in the air lengthens, the intensityof the sunlight is reduced.

As in FIG. 10a , a point P₀ which is located on the Earth's surface andwhich the sunlight reaches in a vertical direction (zenith) and a pointP which is located on the Earth's surface and which the sunlight reachesat an angle φ inclined with respect to the perpendicular are compared (arelationship of the altitude of the sun=90−φ is formed).

The sunlight cast to P₀ moves in the air by H (the thickness of theatmosphere).

Assuming that a distance at which the sunlight cast to P moves in theair is x (a distance between P and Q), (R+H)²=R²+2−2R×cos(180−φ)according to the relationship of the second law of cosines at a triangleOPQ. When this formula is arranged as the quadratic equation of x, thenx²+2(R cos φ)x−(H²+2RH)=0.

Here, R is the radius of the Earth, H is the thickness of theatmosphere, O is the center of the Earth, Q and Q₀ are the (imaginary)points at which the sunlight traveling to P and the sunlight travelingto P₀ meet the outermost shell of the air. Further, it is assumed thatthe Earth is a complete sphere, and the sunlight is not refracted whentraveling in the air. Obtaining a physically meaningful solution of thequadratic equation, x(φ)=(R² cos² φ+H²+2RH)^(1/2)−R cos φ.

Here, when defined as x(φ)/H≡β(φ),

β(φ)=[(R/H)² cos² φ+1+2(R/H)]^(1/2)−(R/H)cos φ. When an incident angleof the sunlight forms the angle φ with respect to the perpendicular, alength of the path of the sunlight in the air is x(φ), and β(φ) is aratio of the length x(φ) of the path of the sunlight in the air to thethickness H of the air.

Assuming that R=6,400 Km and H=100 Km,

β(φ)=[64² cos^(2 φ+)129]^(1/2)−64 cos φ.

In fact, the earth is an ellipsoid having an equatorial radius of 6,378Km and a polar radius of 6,357 Km, but it may be assumed to be a sphere.

The thickness H of the air is not a numerical value that may bedefinitely determined in practice. Since the air has an atmosphericpressure of about 1/100 at an altitude of 30 km and about 1/1,000,000 atan altitude of 100 km, there is no big error even though the thickness Hof the air is assumed to be 100 km. Especially, no matter how thethickness H of the air is set, β(φ) has a similar form. However, if H isgreatly set, a value of β(φ) according to φ is reduced. Assuming thatH=32 Km, β(90°)=20.02. Assuming that H=100 Km, β(φ=90°)=11.36. Assumingthat H=320 Km, β(φ=90°)=6.403.

Next, let's predict how the sunlight is attenuated when passing throughthe air by setting up a simple model and combining the model with anactually measured value. It may be assumed that, when the sunlighttravels along a path dx in the air, a variation di of the intensity isas follows: di/dx=−γ(p, λ)i.

The above formula is one of the differential equations that are mostfrequently and availably applied to analyze phenomena in the naturalworld.

The physical assumption that an attenuation coefficient γ(p, λ) is avariable depending on a wavelength λ of light and on a pressure p iseasily acceptable, because the pressure p is directly associated with analtitude h from the earth's surface and it is directly associated withthe density air molecules if a change in temperature can be ignored.

The above formula is arranged as di/i=−γ(p, λ)dx, and a solution thereofthrough integration, i=I₀ exp[−∫∫(p, λ)dx] is obtained. Here, I₀ is theintensity of the sunlight immediately before the sunlight enters theatmosphere, and an integral of the formula is calculated along the pathof the sunlight in the air.)

A more general solution is given as follows.

i=Σi{I₀ ^(i) exp[−∫γy_(i)(p)dx]} Here, I₀ ^(i) is the intensity of alight component having a wavelength λ_(i) among light components of thesunlight immediately before the sunlight enters the atmosphere, andγ_(i)(p) is an attenuation coefficient of the light component having thewavelength λ_(i) in an environment in which the atmospheric pressure inthe air is p. When measurement is made according to each wavelength bandλ_(i), a model of the intensity of the actual sunlight according to achange in altitude of the sun may be accurately set up. However, sinceit is sufficient only when a rough aspect is grasped, let's furthersimplify the assumption to infer a change in the intensity of thesunlight according to the change in altitude of the sun.

The assumption is as follows. It is assumed that the sunlight consistsof two light components, one of which has a wavelength λ_(A) capable ofreaching the Earth's surface and the other of which has a wavelengthλ_(B) completely attenuated prior to reaching the Earth's surface.Short-wavelength light such as ultraviolet hardly reaches the Earth'ssurface due to absorption of ozone in the upper layer of the air.

In this case, i=I₀ ^(A)exp[−∫γ_(A)(p)dx]+I₀ ^(B) exp[−∫γ_(B)(p)dx].

That is, since γ_(B)(p)>>γ_(A)(p) at least in a partial atmosphericpressure section, I₀ ^(A) (intensity of the light having the wavelengthλ_(A) outside the atmosphere) is attenuated in the air, and only theremaining value thereof reaches the Earth's surface.

Let's substitute the model for the actually measured value to obtainvalues of I₀ ^(A), I₀ ^(B), and ∫γ_(A)(p)dx.

AM0=1353 W/m²: solar constant outside the atmosphere

AM1=925 W/m²: intensity of the sunlight when φ=0°

AM1.5=832 W/m²: intensity of the sunlight when φ45°

AM2=691 W/m²: intensity of the sunlight when φ=60°

In FIG. 10a , AM0 is the intensity of the sunlight which is measured atQ or Q₀ that is the outermost shell of the atmosphere, AM1 is theintensity of the sunlight which is obtained by measuring the sunlightpassing through a path Q₀−P₀ at P₀ on the Earth's surface, and AM1.5 andAM2 are the intensities of the sunlight which are obtained by measuringthe sunlight passing through a path Q−P at P on the Earth's surface(φ=45° and φ=60° respectively).

Before substitution of the actually measured value, let's examine howintegral values ∫γ_(A)(p)dx are mutually related when passing throughthe path Q₀−P₀ (hereinafter referred to as “0 path”) and the path Q−P(hereinafter referred to as “φpath”). Assuming that, when the sunlightpasses through the 0 path (path Q₀−P₀), the integral valueT≡∫_(0 path)γ_(A)(p)dx, consequently a relationship in which, when thesunlight passes through the φpath (path Q−P), the integral value∫_(φ path)γ_(A)(p)dx=T×β(φ) is formed.

The reason is as follows. As illustrated in FIG. 10b , when passingthrough atmospheric layers A1, A2, and A3 having different atmosphericpressures, sunlight AM1 cast in the vertical direction and sunlightAM(φ) obliquely cast at an angle φ with respect to the verticaldirection are attenuated with the same attenuation coefficient γ_(A)(p),and a ratio between the paths in each atmospheric layer is equally1:β(φ). Therefore, a ratio between the integrals, ∫_(0 path)(p)dx and∫_(φ path)(p)dx, calculated according to the different paths is also1:β(φ). That is, a ratio between integral values according to the twopaths is identical to a ratio between lengths of the two paths.

Thus, applying T defined above and the examined result, the followingformula is formed.

AM0=1353=I ₀ ^(A) +I ₀ ^(B)  (Formula 10a)

AM1=925=I ₀ ^(A)exp[−∫_(0 path)γ_(A)(p)dx]=I ₀ ^(A)exp[−T]  (Formula10b)

AM1.5=832=I ₀ ^(A)exp[−∫_(φ(=45°)path)γ_(A)(p)dx]=I ₀^(A)exp[−T×β(φ=45°)]  (Formula 10c)

AM2=691=I ₀ ^(A)exp[−∫_(φ(=60°)path)γ_(A)(p)dx]=I ₀^(A)exp[−T×β(φ=60°)]  (Formula 10d)

It has been previously described that, when the thickness of theatmosphere is assumed to be H=100 Km, the ratio β(φ) of the atmosphericlayer passing length to H according to φ is given as

x(φ)/H≡β(φ)=[64² cos² φ+129]^(1/2)−64 cos φ

A value of β(φ) required to calculate I₀ ^(A), I₀ ^(B), and T is as inTable 2 below.

TABLE 2 φ(degree) β(φ) 0 1 β(60°)-β(0°) 0.955854 45 1.403499β(45°)-β(0°) 0.403499 60 1.955854  β(60°)-β(45°) 0.552355

In the relationship among Formulas 10b, 10c and 10d, T is calculated asin Table 3 below.

TABLE 3 Relationship T Between AM1-AM2 (10b/10d) 0.30512 BetweenAM1-AM1.5 (10b/10c) 0.262606 Between AM1.5-AM2 (10c/10d) 0.336183Average 0.3013

A value of I₀ ^(A) calculated by applying the four values for T toFormulas 10b, 10c and 10d is given as in Table 4 below.

TABLE 4 T = 0.3013 T = 0.30512 T = 0.262606 T = 0.336183 AM1 1250.2493981255.033758 1202.791155 1294.626015 AM1.5 1269.924859 1276.7506351202.791155 1333.637398 AM2 1245.693301 1255.033758 1154.8889161333.637398 Average 1255.289186 1262.272717 1186.823742 1320.633604

Considering that an inference is drawn based on an extremely simplifiedassumption and the nearly qualitative examination, a variation value ofT and a variation value of I₀ ^(A) are not very great. The T value(0.3051) extracted from the relationship of AM1-AM2 and the I₀ ^(A)value (1,255 W/m²) obtained by causing the T value to correspond to AM2are taken to be closest to an average value, and an I₀ ^(B) value (98W/m²) obtained by substituting the I₀ ^(A) value for AMO (Formula 10a)is taken. Since the light component having the wavelength λ_(B) is nottransferred to the Earth's surface, it does not matter that I₀ ^(B) hasany value in the aspect of the photovoltaic power generation.

Consequently, the intensity of the sunlight obliquely cast at the angleφ with respect to the perpendicular may be modeled asi(φ)=1255×exp[−03051×β(φ)] on the earth's surface.

The attenuation coefficient γ(p) that is a function of the pressure p isnot directly used in the above inference.

The calculated results of the atmosphere path length ratio β(φ) and thesunlight intensity model i(φ) according to the incident angle φ are asin Table 5 below. In FIG. 10c , a graph for normalized β(φ) and i(φ) isillustrated.

In the graph of FIG. 10c , i(φ) is denoted by Intensity(φ).

TABLE 5 φ(degree) φ(radian) cosφ β(φ) i(φ) 0 0 1 1 924.9973305 50.0872665 0.9961947 1.0037607 923.9365954 10 0.1745329 0.98480781.0151839 920.722098 15 0.2617994 0.9659258 1.034705 915.2546594 200.3490659 0.9396926 1.0630955 907.3610156 25 0.4363323 0.90630781.1015385 896.7807958 30 0.5235988 0.8660254 1.1517548 883.1459387 350.6108652 0.819152 1.2162048 865.9496635 40 0.6981317 0.76604441.2984124 844.5004157 45 0.7853982 0.7071068 1.4034986 817.8536681 500.8726646 0.6427876 1.5390874 784.7108281 55 0.9599311 0.57357641.7169164 743.270211 60 1.0471976 0.5 1.9558537 691.0138659 65 1.1344640.4226183 2.2879213 624.4342683 70 1.2217305 0.3420201 2.7712254538.8245312 75 1.3089969 0.258819 3.5199027 428.7905374 80 1.39626340.1736482 4.7770645 292.1912235 85 1.4835299 0.0871557 7.0756376144.910141 90 1.5707963 0 11.357817 39.23703138

Although the above model depends on an extremely simplified assumption,the actually measured value is combined to determine the value of aconstant, and thus it may be substantially regarded as an averageintensity of the light components reaching the Earth's surface. It hasalready been mentioned that a more accurate model may be obtained bydivision and actual measurement according to the magnitude λ_(i) of thewavelength. Particularly, it is favorable to more precisely analyze thewavelength band contributed to the photoelectric conversion.

It can be found in FIG. 10c that it is not until φ is between 70° and75° that the intensity of the sunlight is reduced to half. This gives ahint that, however low the altitude of the sun is, the photovoltaicpower generation is sufficiently possible through the uniform lightcondensing using the flat mirrors of the disclosed technology. Ofcourse, if the altitude of the sun is low, a probability of the sunbeing lost in the clouds increases.

Thus, even in the regions where the intensity of the sunlight is so weakthat the photovoltaic power generation has not been persuasive by theconventional method, the photovoltaic power generation may be tried bythe disclosed technology.

Although exemplary embodiments of the disclosed technology have beendescribed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims. The scope of the disclosedtechnology should be limited only by the accompanying claims.

What is claimed is:
 1. A device for photovoltaic power generation,comprising: a frame; a photovoltaic panel disposed on the frame; two ormore flat mirrors disposed on the frame; a rotation mechanism configuredto rotate the frame biaxially to track sunlight; and a supportingstructure configured to support the frame and the rotation mechanism,wherein the photovoltaic panel and the flat mirrors are disposed on theframe to meet a predetermined relationship, said predeterminedrelationship including relations that, in an orthogonal coordinatesystem whose reference lines vary to allow a unit vector in a travelingdirection of sunlight to be (0, 0, 1) when the rotation mechanismnormally tracks the sun, (i) a z-axial component of a unit vector in adirection normal to a light receiving surface of the photovoltaic panelhas a positive value, and (ii) a z-axial component of a unit vector in adirection normal to a reflective surface of each of the flat mirrors hasa negative value to allow the sunlight reflected by each of the flatmirrors to be uniformly incident upon the light receiving surface of thephotovoltaic panel, wherein the photovoltaic panel and the flat mirrorsare disposed on the frame such that projected images of the flat mirrorson an x-y plane of the orthogonal coordinate system are arranged arounda projected image of the photovoltaic panel on the x-y plane of theorthogonal coordinate system in a matrix pattern, wherein an x axislength (M_(x) ^(i,j)) and a y axis length (M_(y) ^(i,j)) of an arbitraryone (M^(i,j)) of the flat mirrors are set to meet the conditions: M_(x)^(i,j)≧p_(x) cos θ_(x) ^(i,j)+δ_(x) ^(i,j) and M_(y) ^(i,j)≧p_(y) cosθ_(y) ^(i,j)+δy^(i,j), where (i) p_(x) and p_(y) represent an x-axislength and a-y ax is length of the photovoltaic panel, respectively,(ii) δ_(x) ^(i,j) and δ_(y) ^(i,j) represent a minimum x-axis marginallength and a minimum y-axis marginal length of one of the flat mirrors(M^(i,j)) respectively, which are required to offset arrangement errorsof the photovoltaic panel and the flat mirror (M^(i,j)), an operationalerror of the rotation mechanism, and vibration to be caused by anexternal force including wind, and (iii) θ_(x) ^(i,j) and θ_(y) ^(i,j)are obtained from the equations: tan 2θ_(x) ^(i,j)=d_(x) ^(i,j)/l^(i,j)and tan 2θ_(y) ^(i,j)=d_(y) ^(i,j)/l^(i,j) where a vector (d_(x) ^(i,j),d_(y) ^(i,j), l^(i,j)) is a displacement vector from a center of thelight receiving surface of the photovoltaic panel to a center of areflective surface of the flat mirror (M^(i,j)) in the orthogonalcoordinate system, and wherein the rotation mechanism is furtherconfigured to rotate the frame according to a change in position of thesun such that the predetermined relationship between the photovoltaicpanel and the flat mirrors is maintained.
 2. The device according toclaim 1, wherein the photovoltaic panel is disposed on the frame suchthat the unit vector in the direction normal to the light receivingsurface of the photovoltaic panel is (0, 0, 1).
 3. The device accordingto claim 1, wherein an optical filter to selectively transmit light of awavelength effective for photoelectric conversion by the photovoltaicpanel is mounted on at least one of the reflective surface of each ofthe flat mirrors or the light receiving surface of the photovoltaicpanel.
 4. The device according to claim 1, wherein the sun trackingsystem comprises first rotating means configured to rotate the frame ina vertical direction and second rotating means configured to rotate theframe in a horizontal direction.
 5. The device according to claim 1,wherein the frame is configured not to block the sunlight on an opticalpath from the sun to the photovoltaic panel via the flat mirrors.
 6. Thedevice according to claim 1, wherein a variable number of flat mirrorsamong the flat mirrors are in use for light condensing according to anintensity of the sunlight before light condensing in order to obtain areflected light condensed to a proper level.
 7. The device according toclaim 1, further comprising a supplementary panel located on a rearsurface of the photovoltaic panel, wherein the supplementary panel isdisposed on the frame such that a unit vector in a direction normal to alight receiving surface of the supplementary panel is (0, 0, −1).
 8. Adevice for photovoltaic power generation, comprising: a frame; aphotovoltaic panel disposed on the frame; two or more flat mirrorsdisposed on the frame; a rotation mechanism configured to rotate theframe biaxially to track sunlight; and a supporting structure configuredto support the frame and the rotation mechanism, wherein thephotovoltaic panel and the flat mirrors are disposed on the frame tomeet a predetermined relationship, said predetermined relationshipincluding a condition that, in an orthogonal coordinate system whosereference lines vary to allow a unit vector in a traveling direction ofsunlight to be (0, 0, 1) when the rotation mechanism normally tracks thesun, (i) a z-axial component of a unit vector in a direction normal to alight receiving surface of the photovoltaic panel has a positive value,and (ii) a z-axial component of a unit vector in a direction normal to areflective surface of each of the flat mirrors has a negative value toallow the sunlight reflected by each of the flat mirrors to be uniformlyincident upon the light receiving surface of the photovoltaic panel,wherein said device further includes a cooling system configured to coolthe photovoltaic panel, said cooling system including a fluidic coolantthat is in direct contact with surfaces of the photovoltaic panel, andwherein the rotation mechanism is further configured to rotate the frameaccording to a change in position of the sun such that the predeterminedrelationship between the photovoltaic panel and the flat mirrors ismaintained.
 9. The device according to claim 8, wherein the photovoltaicpanel is disposed on the frame such that the unit vector in thedirection normal to the light receiving surface of the photovoltaicpanel is (0, 0, 1).
 10. The device according to claim 8, wherein anoptical filter to selectively transmit light of a wavelength effectivefor photoelectric conversion by the photovoltaic panel is mounted on atleast one of the reflective surface of each of the flat mirrors or thelight receiving surface of the photovoltaic panel.
 11. The deviceaccording to claim 8, wherein the sun tracking system comprises firstrotating means configured to rotate the frame in a vertical directionand second rotating means configured to rotate the frame in a horizontaldirection.
 12. The device according to claim 8, wherein the frame isconfigured not to block the sunlight on an optical path from the sun tothe photovoltaic panel via the flat mirrors.
 13. The device according toclaim 8, wherein a variable number of flat mirrors among the flatmirrors are in use for light condensing according to an intensity of thesunlight before light condensing in order to obtain a reflected lightcondensed to a proper level.
 14. The device according to claim 8,further comprising a supplementary panel located on a tear surface ofthe photovoltaic panel, wherein the supplementary panel is disposed onthe frame such that a unit vector in a direction normal to a lightreceiving surface of the supplementary panel is (0, 0, −1).
 15. Thedevice according to claim 8, wherein the cooling system comprises: asealed housing, at least a portion of at least one surface of the sealedhousing being open; a transparent window configured to cover the openportion of the housing for sealing the housing and transmitting incidentlight to be incident upon the light receiving surface of thephotovoltaic panel; and a holder configured to fix the photovoltaicpanel to be disposed within the housing at a predetermined distance froman internal surface of the window, wherein the fluidic coolant is filledin the housing to wrap and cool the photovoltaic panel.
 16. A coolingsystem for cooling a photovoltaic panel, comprising: a sealed housing,at least a portion of at least one surface of the sealed housing beingopen; a transparent window configured to cover the open portion of thehousing for sealing the housing and transmitting incident light to beincident upon a light receiving surface of the photovoltaic panel; aholder configured to fix the photovoltaic panel to be disposed withinthe housing at a predetermined distance from an internal surface of thewindow; and a fluidic coolant filled in the housing to wrap and cool thephotovoltaic panel.
 17. The cooling system according to claim 16,wherein a plurality of valves or holes are provided on a portion of thehousing to allow the fluidic coolant to flow into and out of the housingand allow electric wires to be connected thereto.
 18. The cooling systemaccording to claim 16, further comprising a circulator configured tocause forcible convection of the coolant in the housing.
 19. The coolingsystem according to claim 16, wherein the window and the photovoltaicpanel are removably coupled to the housing and the holder, respectively.20. The cooling system according to claim 16, wherein an optical filterto selectively transmit light of a wavelength effective forphotoelectric conversion by the photovoltaic panel is provided on thewindow of the cooling system.
 21. The cooling system according to claim16, wherein the fluidic coolant of the cooling system comprises anoptical filtering material or dye to selectively transmit light of awavelength effective for photoelectric conversion by the photovoltaicpanel.
 22. The cooling system according to claim 16, wherein the fluidiccoolant is water or refrigerant.
 23. The cooling system according toclaim 22, wherein the water or refrigerant includes an anti-foamingagent to inhibit bubbles from being generated in the cooling water. 24.The cooling system according to claim 22, wherein the water orrefrigerant further includes an anti-freezing solution to prevent thewater or refrigerant from being frozen.